A method of treating an article (e.g. a fiber product) with a water/oil repellent composition comprising an emulsion having a copolymer, which has structural units based on a monomer having a polyfluoroalkyl group (hereinafter referred to as a Rf group) having at least 8 carbon atoms, dispersed in a medium, has been known as a method for imparting water repellency to a surface of the article.
However, recently, EPA (United States Environmental Protection Agency) has pointed out that a compound having a perfluoroalkyl group (hereinafter referred to as a RF group) having at least 8 carbon atoms is decomposed in the environment or a living body, and its decomposition product is accumulated there, which means that such a compound has a major impact on the environment. Accordingly, a copolymer for a water/oil repellent composition, is required which has structural units based on a monomer having a Rf group having at most 6 carbon atoms and which has structural units based on a monomer having a RF group having at least 8 carbon atoms reduced as much as possible.
As a water/oil repellent composition containing such a copolymer, the following water/oil repellent composition has, for example, been proposed.
A water/oil repellent composition essentially containing a copolymer which substantially comprises structural units based on the following monomer (A) and structural units based on the following monomer (B) (Patent Document 1):
Monomer (A): e.g. a monomer which has a RF group having at most 6 carbon atoms;
Monomer (B): e.g. a (meth)acrylate which has an alkyl group having at least 15 carbon atoms.
However, an article treated with such a water/oil repellent composition is insufficient in water repellency to water (rainfall) fallen from a high altitude (hereinafter referred to as dynamic water repellency), water repellency after drying without compulsory heating after washing (hereinafter referred to as after air-drying water repellency) and water repellency after friction (hereinafter referred to as friction durability).    Patent Document 1: WO02/083809